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Laser-induced vibration of a thin soap film Cite this: Lab Chip, 2014, 14, 3525
Olivier Emile†*a and Janine Emile†b We report on the vibration of a thin soap film based on the optical radiation pressure force. The modulated low power laser induces a counter gravity flow in a vertical free-standing draining film. The thickness of the soap film is then higher in the upper region than in the lower region of the film. Moreover, the lifetime
Received 28th May 2014, Accepted 30th June 2014 DOI: 10.1039/c4lc00626g
of the film is dramatically increased by a factor of 2. Since the laser beam only acts mechanically on the film interfaces, such a film can be employed in an optofluidic diaphragm pump, the interfaces behaving like a vibrating membrane and the liquid in-between being the fluid to be pumped. Such a pump could then be used in delicate micro-equipment, in chips where temperature variations are detrimental and even in
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biological systems.
1. Introduction Optofluidics is a research domain that takes advantages of microfluidics and optics to synthesize novel functionalities for applications that include biophotonic systems, lab-onchip devices, biosensors and molecular imaging.1–6 One of the key issues in incorporating photonic systems into microfluidic devices is the realization of microvalves and micropumps.6,7 Most of the vibrating mechanisms of pumping devices reported up to now mostly relies on local optical heating that induces a flow.8–12 This heating may become detrimental when using delicate integrated microsystems or biological devices or cells. On the other hand, the optical radiation pressure can induce a pressure force on air/liquid or liquid/liquid interfaces.13,14 However, it is quite difficult to bend or deform interfaces using light only. People use either quite high power lasers,13 or low power lasers with liquids which have similar surface tensions.15 Indeed, low power radiation pressure deformation of air/liquid interfaces is hardly noticeable except under total internal reflection conditions close to the critical angle.16 Nevertheless, parametric amplification or resonance may dramatically enhance this phenomenon in general and may thus lead to a higher surface deformation of the interfaces. Then, considering a soap film as a model system, since a single laser would act on both air/liquid interfaces, the soap film may then vibrate in a symmetrical manner, like a balloon that inflates and deflates under the influence of the laser. One may wonder what consequences such a vibrating liquid membrane may induce in the free drainage of the film and whether it then can be adapted to be used as a diaphragm pump in optofluidic a
URU 435 Physique des Lasers, Université de Rennes 1, 35042 Rennes Cedex, France. E-mail: [email protected]; Tel: 33 2 23 23 65 21 b IPR, UMR CNRS 6251, Université de Rennes 1, 35042 Rennes Cedex, France † Both authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2014
devices. Such a pump would be easy to implement, versatile, since it could be adapted to any fluid and even to soft materials, and low cost, since it could be implemented with any chopped light source. The aim of this article is precisely to look for the response of a vertical free-standing draining soap film under the influence of a modulated laser and to investigate its lifetime and thickness variations.
2. Experimental set-up The experimental set-up is sketched in Fig. 1. A vertical freestanding draining soap film supported by a glass frame experiences a modulated radiation pressure force from a chopped green solid-state laser, hereafter referred to as “modulated laser” (Cristal Laser, P = 100 mW, attenuated to P = 2 mW, waist w = 300 μm, λ = 532 nm). Although much more accurate techniques exist to measure thin film thickness variations,17 we use the interference fringes in transmission from two
Fig. 1 Experimental set-up. Laser light from a 532 nm, −2 mW solidstate laser deforms a vertical free-standing thin soap film. The film thickness is measured from the transmission interferences of two low power lasers with different colors (λ = 633 nm and λ = 543 nm). BS: beam splitter, Dg and Dr: photodiodes detecting green and red light, respectively.
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continuous wave (CW) low power He–Ne lasers (Mlles Griot, P = 1 mW, w = 400 μm, λ = 633 nm and λ = 543 nm, respectively) in order to measure the absolute film thickness.18 These lasers will be called “probe beams” hereafter. The film thickness is deduced from the relative position of the intensity maxima and minima of the transmission of the two lasers with an absolute precision of the order of 10 nm. Indeed, this technique is quite easy to handle and versatile, and fast acquisition times can be obtained. We have checked that when the solid state laser is not modulated or is not applied, the lifetime and the drainage curve of the liquid film are exactly alike. Thus, the influence of the two low power CW probe beams on the film dynamics could be neglected. The modulated beam was attenuated below 2 mW in order to avoid any deformation of the film interfaces due to higher laser power such as thermal or Marangoni effects.17,19 The glass frame supporting the film is a 3 cm diameter toroidal ring. The glass rod diameter is 5 mm. The soap solution is composed of a well-known surfactant, SLES (sodium lauryl ether sulfate, Cognis), 0.1% (v/v) in pure water, below the critical micellar concentration (CMC). Immersing the frame in the soap solution produces the film. The frame is then placed vertically in front of the lasers. The film is centered on the modulated beam. The temperature is controlled to T = 20.0 ± 0.2 °C, and the air humidity to 50 ± 5%. The two transmitted probe laser signals are recorded on two photodiodes and then registered on a computer. The acquisition rate can be changed from 1 ms to 0.1 s.
3. Results 3.1. Dynamical response Let us first investigate the dynamical response of the film under sudden laser excitation. For this purpose, we have switched the laser on and off with a mechanical shutter. We have recorded the response of the film (thickness variation) via the intensity transmitted by the probe laser beams, as can be seen in Fig. 2. First of all, one can notice that the film shows a mechanical response under laser excitation. The film gets thinner as the laser is switched on and thicker as the laser is switched off, as can be expected from the usual radiation pressure force.13,14 One has to note that both interfaces are bent due to the radiation pressure. When the laser is on and not modulated, the soap film experiences a permanent extra stress that leads to a tiny deformation hardly noticeable on the global drainage curves. The stress is released when the laser is off. Moreover, when the modulated laser and the CW lasers are not perfectly aligned, one can see, when switching on the modulated laser, a tiny bump and then a dip in the film thickness versus time, meaning that we can easily generate travelling waves on the film interfaces. Typically, when the laser is switched on, the time response of the film is 0.1 s with a thinning of 20 nm. The reverse effect is observed when the laser is switched off, with the same characteristics
3526 | Lab Chip, 2014, 14, 3525–3529
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Fig. 2 Deformation of the film interfaces. Interference transmission fringes during the drainage of the film, while the modulated laser is switched on and off several times (a). n.u.: normalized units. Zoom on the fringes for two specific cases: (b) laser off and (c) laser on. The dotted lines are a prolongation of the evolution of the fringes without the applied perturbation. We estimate that the interface deformation is of the order of 20 nm when the laser is either on or off.
(see Fig. 2). Since the CW probe lasers are always used on the film, they may have a little effect on the absolute thickness of the film but not on the relative variation (drainage curve), as mentioned previously. Indeed, the effects we discussed are tiny deformations. However, could they modify the flow inside the film or even reverse it by modulating them? 3.2. Resonance Since the response time of the soap film is of the order of 0.1 s, the optimal modulation frequency should be of the order of 10 Hz. This is also the typical resonance frequency range of soap film vibrations excited by acoustic waves.20–22 Let us chop the beam at a low frequency and look at the lifetime of the film, i.e. the time before its rupture. Such a response is sketched on Fig. 3. One can see a quite sharp resonance around a frequency of 23.5 Hz. The quality factor of the resonance is equal to Q = 8, which seems to be rather
Fig. 3 Lifetime of the film versus modulation frequency. A resonance behaviour of the lifetime of the film for around 23.5 Hz frequency modulation of the laser beam is evidenced. The quality factor Q equals 8. The lifetime of the film is more than doubled at resonance.
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high for such a mechanical system. Outside the resonance, at high frequencies, the film does not experience the modulation and thus exhibits CW excitation. At low frequencies, below resonance, the film reaches a steady state before experiencing the following excitation, as shown in Fig. 2. In order to get some more physical insights, one can try to modify the film stiffness. For example, above the CMC, the film structure is changed by micellar stratification.17 The film should be stiffer. We have thus observed an increase in the resonance frequency by increasing CMC (for example, 25 Hz at 2 CMC). One can also tune the bulk viscosity and the surface shear viscosity by adding some glycerol.23 The damping coefficient of the film increases, leading to a decrease in the resonance frequency (22 Hz for 5 wt% glycerol added). The quality factor also decreases to Q = 5. These results are globally in qualitative agreement with results observed in acoustic studies.21,22 Here, the amplitude of the vibrations is much lower than that of acoustic waves. In their case, they observed with the naked eye the white light interferences due to the film thickness to evidence the vibrating modes, which is not possible here. However, these are the two main differences between acoustic excitation and laser excitation. First, the acoustic excitation is induced via a loudspeaker in order to have a uniform vibration. Here, the excitation is point like. Second, bending or antisymmetric modes24,25 are preferentially excited with acoustics,22 whereas symmetric or squeezing modes are observed here, mainly because the radiation pressure acts on both air/liquid interfaces symmetrically. On the basis of these studies, one would also expect a lower resonance frequency for a frame with higher dimensions. However, we find an increase in the resonance frequency (for example, 33 Hz for a 4 cm diameter frame). Actually, the studied film is a vertical free-standing film, whereas experiments with sound-induced vibrations used horizontal films. The modulated laser has to counterbalance gravity. The effect of gravity is even more disturbing with a larger frame since the liquid volume excited by the modulated laser is higher, leading to a higher resonance frequency. It follows that the resonance frequency can thus be adapted to the physicochemical properties of the liquid film and to the geometry of the frame.
4. Drainage curves Since the lifetime of the soap film can be dramatically increased, one could wonder how the drainage curves and the film thickness are then modified under modulated excitation. We have moved the probe beams in order to measure the film thickness at 3 different positions (see Fig. 4). First, we probed the film in the middle of the frame where the modulated laser hits the film, then 0.75 cm above and 0.75 cm below, away from the edges of the frame. In the absence of the modulated laser, one gets the usual drainage curves23–27 with a t −1 dependence on the middle and on the upper part of the film, t being the time. Gravity and
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Fig. 4 Drainage curves of the film without and with the laser modulation at resonance. Thickness evolution of the film versus time at the top, middle and bottom of the frame, without and with the laser modulation, on a log–log scale. The laser modulation frequency equals 23.5 Hz. The red points are experimental values together with the error bars due to the statistics; each measurement was performed three times. The black solid line is the power fit. The absolute precision on the power law is 0.03.
evaporation are the two main mechanisms governing the drainage. In the lower part of the film, due to the presence of the meniscus and related to the accumulation of liquid by gravity, the power law thinning is smoother with the t −0.5 law. In the presence of the modulated laser, the situation is dramatically changed. As already shown in Fig. 2, the lifetime of the film is increased by more than a factor of 2. One can see from Fig. 4 that the power law thinning in the middle of the film is unchanged; only the final thickness decreases greatly due to the longer lifetime (about 35% thinner). In the bottom of the film, the power law changes from t −0.5 to t −1, although at the end of the drainage, there is a small departure from this power law. The final thickness also decreases greatly (about 80% thinner). Indeed, the liquid feeding of this zone from the upper region is dramatically reduced. On the contrary, at the top of the film, the power law changes from t −1 to t −0.15. The final thickness significantly increases by nearly a factor of 3, and the film hardly drains in this region.
5. Discussion and applications The drainage evolution is thus globally reversed with an exchange between the top and the bottom of the film. Let us try to investigate the flows inside the film. At the beginning of the drainage, the film is thicker at the bottom than at the top (see Fig. 4). When the laser is switched on, it creates a little dip on both sides of the film, flushing the liquid outward. On the bottom of the film, the excess of liquid is evacuated down towards the meniscus due to gravity (see Fig. 5). On the top of the film, the liquid runs uphill and experiences a new laser flush while it runs downhill towards the center of the
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6. Conclusions
Fig. 5 Influence of the laser on the interfaces (green arrow), flow of the liquid in the film (red arrow) and influence of gravity (blue arrow). The deformations are exaggerated. Left drawing and dotted lines: freestanding draining film. Other drawings: laser on–off cycle. In the upper part of the film, the effect of gravity is hidden by the effect of the laser, leading to inflation in the upper zone.
film. The liquid cannot be drained until the meniscus; it thus accumulates in the upper region of the film and evaporates. One thus gets a film that is thinner in the lower region than in the upper region. The effect is amplified at the end of the drainage. Since the film is thinner at the bottom, the pressure is higher. When the laser is switched on, the liquid at the center flushes uphill. When the laser is switched off, the liquid flows from the lower region toward the center (see Fig. 5). The liquid is thus pumped from the lower zone towards the upper zone. The pressure difference is indeed acting as a valve. We have been able to make the flow run uphill8 with a pure mechanical effect. This effect could also explain the departure from the power-law behaviour t −1 of the thickness evolution of the lower part of the film (see Fig. 4). In that region, the liquid is drained downhill due to gravity and flushed uphill due to the pumping effect. Since micrometer size valves28,29 and photoinduced valves30 have already been demonstrated, this optofluidic pump can be readily implemented for practical applications. These applications are of course not limited to liquid films with air/liquid interfaces. It could be used not only in soap films deposited on a substrate, but also more generally in liquid/liquid interfaces with different optical indexes. It could also be helpful in controlling and regulating the water density or water flow in biological and cell applications. It can even be implemented for liquid core/polymers cladding waveguides.31 It can thus become the dynamical element in optofluidic devices for drug and food flow delivery in in vitro cell cultures. Besides, since the modulated laser is a very low power laser (2 mW output power), low-consumption cheap diode laser or even natural light from the sun can also be used; thus this optofluidic pump would be of valuable help in remote areas with limited energy supply,32,33 where people can rely on solar energy.
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We have experimentally shown that even a low power laser can dramatically change the dynamical behaviour of a liquid draining film. The action of a modulated laser beam on a vertical soap film flushes the liquid upstream, creating then a bottleneck that reverses the liquid flow from downhill to uphill. This results in a dramatic increase in the lifetime of the film for a given resonance frequency. The power law drainage time evolution and the final thickness of the film are also greatly changed. Such an optofluidic diaphragm pump can be readily implemented in practical devices. For example, such a modulated laser light can be directly shined on the membrane of biological cells to pump liquid inside or outside the cell and may thus find applications in dermatology to remove angioma or in ophthalmology to remove floaters. Since the action of the laser only leads to tiny mechanical vibrations, there would be no biocompatibility problems when used in biomedical and neuronal applications.34,35 Besides, since the optical power is very low, there would be no problem of photothermal damage36–38 due to the laser radiation.
Acknowledgements This work has been performed within the European COST action MP 1205 Advances in optofluidics: integration of optical control and photonics with microfluidics.
References 1 D. Psaltis, S. R. Quake and C. Yang, Developing optofluidic technology through the fusion of microfluidics and optics, Nature, 2006, 442, 381–386. 2 C. Monat, P. Domachuk and B. J. Eggleton, Integrated optofluidics: a new river of light, Nat. Photonics, 2007, 1, 106–114. 3 Y. Fainman, L. P. Lee, D. Psaltis and C. Yang, Optofluidics fundamentals, devices and applications, McGraw-Hill, Montreal, 2010. 4 X. D. Fan and I. M. White, Optofluidic microsystems for chemical and biological analysis, Nat. Photonics, 2011, 5, 591–597. 5 H. Schmidt and A. R. Hawkins, Photonics integration of non-solid media using optofluidics, Nat. Photonics, 2011, 5, 598–604. 6 L. Pang, H. M. Chen, L. M. Freeman and Y. Fainman, Optofluidic devices and applications in photonics, sensing and imaging, Lab Chip, 2012, 12, 3543–3551. 7 L. Chen, S. Lee, S. Choo and E. K. Lee, Continuous dynamic flow micropumps for microfluidic manipulation, J. Micromech. Microeng., 2008, 18, 013001. 8 M. K. Chauhury and G. M. Whitesides, How to make water run uphill, Science, 1992, 256, 1539–1541.
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9 G. L. Liu, J. KIm, Y. Lu and L. P. Lee, Optofluidic control using photothermal nanoparticles, Nat. Mater., 2006, 5, 27–32. 10 F. M. Weinert, J. A. Kraus, T. Franosch and D. Braun, Microscale Fluid Flow Induced by Thermoviscous Expansion Along a Traveling Wave, Phys. Rev. Lett., 2008, 100, 164501. 11 D. A. Boyd, J. R. Adelman, D. G. Goodwin and D. Psaltis, Chemical Separations by Bubble Assisted Interphase MassTransfer, Anal. Chem., 2008, 80, 2452–2456. 12 J. S. Danou, G. Baffou, D. McCloskey and R. Quidant, Plasmon assisted optofluidics, ACS Nano, 2011, 5, 5457–5462. 13 A. Ashkin and J. M. Dziedzic, Radiation Pressure on a Free Liquid Surface, Phys. Rev. Lett., 1973, 30, 139–142. 14 A. Ashkin, Applications of laser radiation pressure, Science, 1980, 210, 181–188. 15 A. Casner and J. P. Delville, Giant Deformations of a LiquidLiquid Interface Induced by the Optical Radiation Pressure, Phys. Rev. Lett., 2001, 87, 054503. 16 O. Emile and J. Emile, Low-Power Laser Deformation of an Air-Liquid Interface, Phys. Rev. Lett., 2011, 106, 183904. 17 J. Emile and O. Emile, Mapping of the Marangoni effect in soap films using Young's double-slit experiment, EPL, 2013, 104, 14001. 18 J. Emile, F. Casanova, G. Loas and O. Emile, Swelling of a foam lamella in a confined channel, Soft Matter, 2012, 8, 7223. 19 S. Chandrasekhar, Hydrodynamic and hydromagnetic, Dover, New-York, 1981. 20 E. B. Tylor, Sound vibrations of soap film membranes, Nature, 1877, 16, 12–12. 21 L. Bergmann, Experiments with vibrating soap membranes, J. Acoust. Soc. Am., 1956, 28, 1043–1047. 22 A. Boudaoud, Y. Couder and M. Ben Amar, Self adaptation in vibrating soap films, Phys. Rev. Lett., 1999, 82, 3847–3850. 23 S. Berg, E. A. Adelizzi and S. M. Troian, Experimental Study of Entrainment and Drainage Flows in Microscale Soap Films, Langmuir, 2005, 21, 3867–3876. 24 P. Sens, C. Marques and J. F. Joanny, Hydrodynamics modes of viscoelastic soap films, Langmuir, 1993, 9, 3212–3218. 25 C.-Y. D. Lu and M. E. Cates, Hydrodynamic modes of soluble surfactant films, Langmuir, 1995, 11, 4225–4233.
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26 R. J. Braun and A. D. Fitt, Modelling drainage of the precorneal tear film after a blink, Math. Med. Biol., 2003, 20, 1–28. 27 S. N. Tan, Y. Yang and R. G. Horn, Thinning of a Vertical Free-Draining Aqueous Film Incorporating Colloidal Particles, Langmuir, 2010, 26, 63–73. 28 V. Studer, G. Hang, A. Pandolfi, M. Ortiz, W. F. Anderson and S. R. Quake, Scaling properties of a low-actuation pressure microfluidic valve, J. Appl. Phys., 2004, 95, 393–398. 29 C. Murray, D. McCoul, E. Sollier, T. Ruggiero, X. Niu, O. Pei and D. Di Carlo, Electro-adaptive microfluidics for active tuning of channel geometry using polymer actuators, Microfluid. Nanofluid., 2013, 14, 345–358. 30 F. Benito-Lopez, R. Byrne, A. A. Raduta, N. E. Vrana, G. McGuiness and D. Diamond, Ionogel-Based LightActuated Valves for Controlling Liquid Flow in Microfluidic Manifolds, Lab Chip, 2010, 10, 195–201. 31 P. Fei, Z. Chen, Y. Men, A. Li, Y. Shenac and Y. Huang, A compact optofluidic cytometer with integrated liquid-core/ PDMS-cladding waveguides, Lab Chip, 2012, 12, 3700–3706. 32 L. Jiang, M. Mancuso, Z. Lu, G. Akar, E. Cesarman and D. Erickson, Solar thermal polymerase chain reaction for smartphone-assisted molecular diagnostics, Sci. Rep., 2014, 4, 4137. 33 S. Lee, V. Oncescu, M. Mancuso, S. Mehta and D. Erickson, A smartphone platform for the quantification of vitamin D levels, Lab Chip, 2014, 14, 1437–1442. 34 M. A. Unger, H. P. Chou, T. Thorsen, A. Scherer and S. R. Quake, Monolithic microfabricated valves and pumps by multilayer soft lithography, Science, 2000, 288, 113–116. 35 J. W. Park, H. J. Kim, M. W. Kang and N. L. Jeon, Advances in microfluidics-based experimental methods for neuroscience research, Lab Chip, 2013, 13, 509–521. 36 E. K. Scackmann, A. L. Fulton and D. J. Beebe, The present and future role of microfluidics in biomedical research, Nature, 2014, 507, 181–189. 37 V. VanDelinder and G. D. Bachand, Photodamage and the importance of photoprotection in biomolecular-powered device applications, Anal. Chem., 2014, 86, 721–728. 38 B. Dura, Y. Liu and J. Voldman, Deformability based microfluidic cell pairing and fusion, Lab Chip, 2014, 14, 2783–2790.
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Laser-induced vibration of a thin soap film Cite this: Lab Chip, 2014, 14, 3525
Olivier Emile†*a and Janine Emile†b We report on the vibration of a thin soap film based on the optical radiation pressure force. The modulated low power laser induces a counter gravity flow in a vertical free-standing draining film. The thickness of the soap film is then higher in the upper region than in the lower region of the film. Moreover, the lifetime
Received 28th May 2014, Accepted 30th June 2014 DOI: 10.1039/c4lc00626g
of the film is dramatically increased by a factor of 2. Since the laser beam only acts mechanically on the film interfaces, such a film can be employed in an optofluidic diaphragm pump, the interfaces behaving like a vibrating membrane and the liquid in-between being the fluid to be pumped. Such a pump could then be used in delicate micro-equipment, in chips where temperature variations are detrimental and even in
www.rsc.org/loc
biological systems.
1. Introduction Optofluidics is a research domain that takes advantages of microfluidics and optics to synthesize novel functionalities for applications that include biophotonic systems, lab-onchip devices, biosensors and molecular imaging.1–6 One of the key issues in incorporating photonic systems into microfluidic devices is the realization of microvalves and micropumps.6,7 Most of the vibrating mechanisms of pumping devices reported up to now mostly relies on local optical heating that induces a flow.8–12 This heating may become detrimental when using delicate integrated microsystems or biological devices or cells. On the other hand, the optical radiation pressure can induce a pressure force on air/liquid or liquid/liquid interfaces.13,14 However, it is quite difficult to bend or deform interfaces using light only. People use either quite high power lasers,13 or low power lasers with liquids which have similar surface tensions.15 Indeed, low power radiation pressure deformation of air/liquid interfaces is hardly noticeable except under total internal reflection conditions close to the critical angle.16 Nevertheless, parametric amplification or resonance may dramatically enhance this phenomenon in general and may thus lead to a higher surface deformation of the interfaces. Then, considering a soap film as a model system, since a single laser would act on both air/liquid interfaces, the soap film may then vibrate in a symmetrical manner, like a balloon that inflates and deflates under the influence of the laser. One may wonder what consequences such a vibrating liquid membrane may induce in the free drainage of the film and whether it then can be adapted to be used as a diaphragm pump in optofluidic a
URU 435 Physique des Lasers, Université de Rennes 1, 35042 Rennes Cedex, France. E-mail: [email protected]; Tel: 33 2 23 23 65 21 b IPR, UMR CNRS 6251, Université de Rennes 1, 35042 Rennes Cedex, France † Both authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2014
devices. Such a pump would be easy to implement, versatile, since it could be adapted to any fluid and even to soft materials, and low cost, since it could be implemented with any chopped light source. The aim of this article is precisely to look for the response of a vertical free-standing draining soap film under the influence of a modulated laser and to investigate its lifetime and thickness variations.
2. Experimental set-up The experimental set-up is sketched in Fig. 1. A vertical freestanding draining soap film supported by a glass frame experiences a modulated radiation pressure force from a chopped green solid-state laser, hereafter referred to as “modulated laser” (Cristal Laser, P = 100 mW, attenuated to P = 2 mW, waist w = 300 μm, λ = 532 nm). Although much more accurate techniques exist to measure thin film thickness variations,17 we use the interference fringes in transmission from two
Fig. 1 Experimental set-up. Laser light from a 532 nm, −2 mW solidstate laser deforms a vertical free-standing thin soap film. The film thickness is measured from the transmission interferences of two low power lasers with different colors (λ = 633 nm and λ = 543 nm). BS: beam splitter, Dg and Dr: photodiodes detecting green and red light, respectively.
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continuous wave (CW) low power He–Ne lasers (Mlles Griot, P = 1 mW, w = 400 μm, λ = 633 nm and λ = 543 nm, respectively) in order to measure the absolute film thickness.18 These lasers will be called “probe beams” hereafter. The film thickness is deduced from the relative position of the intensity maxima and minima of the transmission of the two lasers with an absolute precision of the order of 10 nm. Indeed, this technique is quite easy to handle and versatile, and fast acquisition times can be obtained. We have checked that when the solid state laser is not modulated or is not applied, the lifetime and the drainage curve of the liquid film are exactly alike. Thus, the influence of the two low power CW probe beams on the film dynamics could be neglected. The modulated beam was attenuated below 2 mW in order to avoid any deformation of the film interfaces due to higher laser power such as thermal or Marangoni effects.17,19 The glass frame supporting the film is a 3 cm diameter toroidal ring. The glass rod diameter is 5 mm. The soap solution is composed of a well-known surfactant, SLES (sodium lauryl ether sulfate, Cognis), 0.1% (v/v) in pure water, below the critical micellar concentration (CMC). Immersing the frame in the soap solution produces the film. The frame is then placed vertically in front of the lasers. The film is centered on the modulated beam. The temperature is controlled to T = 20.0 ± 0.2 °C, and the air humidity to 50 ± 5%. The two transmitted probe laser signals are recorded on two photodiodes and then registered on a computer. The acquisition rate can be changed from 1 ms to 0.1 s.
3. Results 3.1. Dynamical response Let us first investigate the dynamical response of the film under sudden laser excitation. For this purpose, we have switched the laser on and off with a mechanical shutter. We have recorded the response of the film (thickness variation) via the intensity transmitted by the probe laser beams, as can be seen in Fig. 2. First of all, one can notice that the film shows a mechanical response under laser excitation. The film gets thinner as the laser is switched on and thicker as the laser is switched off, as can be expected from the usual radiation pressure force.13,14 One has to note that both interfaces are bent due to the radiation pressure. When the laser is on and not modulated, the soap film experiences a permanent extra stress that leads to a tiny deformation hardly noticeable on the global drainage curves. The stress is released when the laser is off. Moreover, when the modulated laser and the CW lasers are not perfectly aligned, one can see, when switching on the modulated laser, a tiny bump and then a dip in the film thickness versus time, meaning that we can easily generate travelling waves on the film interfaces. Typically, when the laser is switched on, the time response of the film is 0.1 s with a thinning of 20 nm. The reverse effect is observed when the laser is switched off, with the same characteristics
3526 | Lab Chip, 2014, 14, 3525–3529
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Fig. 2 Deformation of the film interfaces. Interference transmission fringes during the drainage of the film, while the modulated laser is switched on and off several times (a). n.u.: normalized units. Zoom on the fringes for two specific cases: (b) laser off and (c) laser on. The dotted lines are a prolongation of the evolution of the fringes without the applied perturbation. We estimate that the interface deformation is of the order of 20 nm when the laser is either on or off.
(see Fig. 2). Since the CW probe lasers are always used on the film, they may have a little effect on the absolute thickness of the film but not on the relative variation (drainage curve), as mentioned previously. Indeed, the effects we discussed are tiny deformations. However, could they modify the flow inside the film or even reverse it by modulating them? 3.2. Resonance Since the response time of the soap film is of the order of 0.1 s, the optimal modulation frequency should be of the order of 10 Hz. This is also the typical resonance frequency range of soap film vibrations excited by acoustic waves.20–22 Let us chop the beam at a low frequency and look at the lifetime of the film, i.e. the time before its rupture. Such a response is sketched on Fig. 3. One can see a quite sharp resonance around a frequency of 23.5 Hz. The quality factor of the resonance is equal to Q = 8, which seems to be rather
Fig. 3 Lifetime of the film versus modulation frequency. A resonance behaviour of the lifetime of the film for around 23.5 Hz frequency modulation of the laser beam is evidenced. The quality factor Q equals 8. The lifetime of the film is more than doubled at resonance.
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high for such a mechanical system. Outside the resonance, at high frequencies, the film does not experience the modulation and thus exhibits CW excitation. At low frequencies, below resonance, the film reaches a steady state before experiencing the following excitation, as shown in Fig. 2. In order to get some more physical insights, one can try to modify the film stiffness. For example, above the CMC, the film structure is changed by micellar stratification.17 The film should be stiffer. We have thus observed an increase in the resonance frequency by increasing CMC (for example, 25 Hz at 2 CMC). One can also tune the bulk viscosity and the surface shear viscosity by adding some glycerol.23 The damping coefficient of the film increases, leading to a decrease in the resonance frequency (22 Hz for 5 wt% glycerol added). The quality factor also decreases to Q = 5. These results are globally in qualitative agreement with results observed in acoustic studies.21,22 Here, the amplitude of the vibrations is much lower than that of acoustic waves. In their case, they observed with the naked eye the white light interferences due to the film thickness to evidence the vibrating modes, which is not possible here. However, these are the two main differences between acoustic excitation and laser excitation. First, the acoustic excitation is induced via a loudspeaker in order to have a uniform vibration. Here, the excitation is point like. Second, bending or antisymmetric modes24,25 are preferentially excited with acoustics,22 whereas symmetric or squeezing modes are observed here, mainly because the radiation pressure acts on both air/liquid interfaces symmetrically. On the basis of these studies, one would also expect a lower resonance frequency for a frame with higher dimensions. However, we find an increase in the resonance frequency (for example, 33 Hz for a 4 cm diameter frame). Actually, the studied film is a vertical free-standing film, whereas experiments with sound-induced vibrations used horizontal films. The modulated laser has to counterbalance gravity. The effect of gravity is even more disturbing with a larger frame since the liquid volume excited by the modulated laser is higher, leading to a higher resonance frequency. It follows that the resonance frequency can thus be adapted to the physicochemical properties of the liquid film and to the geometry of the frame.
4. Drainage curves Since the lifetime of the soap film can be dramatically increased, one could wonder how the drainage curves and the film thickness are then modified under modulated excitation. We have moved the probe beams in order to measure the film thickness at 3 different positions (see Fig. 4). First, we probed the film in the middle of the frame where the modulated laser hits the film, then 0.75 cm above and 0.75 cm below, away from the edges of the frame. In the absence of the modulated laser, one gets the usual drainage curves23–27 with a t −1 dependence on the middle and on the upper part of the film, t being the time. Gravity and
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Fig. 4 Drainage curves of the film without and with the laser modulation at resonance. Thickness evolution of the film versus time at the top, middle and bottom of the frame, without and with the laser modulation, on a log–log scale. The laser modulation frequency equals 23.5 Hz. The red points are experimental values together with the error bars due to the statistics; each measurement was performed three times. The black solid line is the power fit. The absolute precision on the power law is 0.03.
evaporation are the two main mechanisms governing the drainage. In the lower part of the film, due to the presence of the meniscus and related to the accumulation of liquid by gravity, the power law thinning is smoother with the t −0.5 law. In the presence of the modulated laser, the situation is dramatically changed. As already shown in Fig. 2, the lifetime of the film is increased by more than a factor of 2. One can see from Fig. 4 that the power law thinning in the middle of the film is unchanged; only the final thickness decreases greatly due to the longer lifetime (about 35% thinner). In the bottom of the film, the power law changes from t −0.5 to t −1, although at the end of the drainage, there is a small departure from this power law. The final thickness also decreases greatly (about 80% thinner). Indeed, the liquid feeding of this zone from the upper region is dramatically reduced. On the contrary, at the top of the film, the power law changes from t −1 to t −0.15. The final thickness significantly increases by nearly a factor of 3, and the film hardly drains in this region.
5. Discussion and applications The drainage evolution is thus globally reversed with an exchange between the top and the bottom of the film. Let us try to investigate the flows inside the film. At the beginning of the drainage, the film is thicker at the bottom than at the top (see Fig. 4). When the laser is switched on, it creates a little dip on both sides of the film, flushing the liquid outward. On the bottom of the film, the excess of liquid is evacuated down towards the meniscus due to gravity (see Fig. 5). On the top of the film, the liquid runs uphill and experiences a new laser flush while it runs downhill towards the center of the
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6. Conclusions
Fig. 5 Influence of the laser on the interfaces (green arrow), flow of the liquid in the film (red arrow) and influence of gravity (blue arrow). The deformations are exaggerated. Left drawing and dotted lines: freestanding draining film. Other drawings: laser on–off cycle. In the upper part of the film, the effect of gravity is hidden by the effect of the laser, leading to inflation in the upper zone.
film. The liquid cannot be drained until the meniscus; it thus accumulates in the upper region of the film and evaporates. One thus gets a film that is thinner in the lower region than in the upper region. The effect is amplified at the end of the drainage. Since the film is thinner at the bottom, the pressure is higher. When the laser is switched on, the liquid at the center flushes uphill. When the laser is switched off, the liquid flows from the lower region toward the center (see Fig. 5). The liquid is thus pumped from the lower zone towards the upper zone. The pressure difference is indeed acting as a valve. We have been able to make the flow run uphill8 with a pure mechanical effect. This effect could also explain the departure from the power-law behaviour t −1 of the thickness evolution of the lower part of the film (see Fig. 4). In that region, the liquid is drained downhill due to gravity and flushed uphill due to the pumping effect. Since micrometer size valves28,29 and photoinduced valves30 have already been demonstrated, this optofluidic pump can be readily implemented for practical applications. These applications are of course not limited to liquid films with air/liquid interfaces. It could be used not only in soap films deposited on a substrate, but also more generally in liquid/liquid interfaces with different optical indexes. It could also be helpful in controlling and regulating the water density or water flow in biological and cell applications. It can even be implemented for liquid core/polymers cladding waveguides.31 It can thus become the dynamical element in optofluidic devices for drug and food flow delivery in in vitro cell cultures. Besides, since the modulated laser is a very low power laser (2 mW output power), low-consumption cheap diode laser or even natural light from the sun can also be used; thus this optofluidic pump would be of valuable help in remote areas with limited energy supply,32,33 where people can rely on solar energy.
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We have experimentally shown that even a low power laser can dramatically change the dynamical behaviour of a liquid draining film. The action of a modulated laser beam on a vertical soap film flushes the liquid upstream, creating then a bottleneck that reverses the liquid flow from downhill to uphill. This results in a dramatic increase in the lifetime of the film for a given resonance frequency. The power law drainage time evolution and the final thickness of the film are also greatly changed. Such an optofluidic diaphragm pump can be readily implemented in practical devices. For example, such a modulated laser light can be directly shined on the membrane of biological cells to pump liquid inside or outside the cell and may thus find applications in dermatology to remove angioma or in ophthalmology to remove floaters. Since the action of the laser only leads to tiny mechanical vibrations, there would be no biocompatibility problems when used in biomedical and neuronal applications.34,35 Besides, since the optical power is very low, there would be no problem of photothermal damage36–38 due to the laser radiation.
Acknowledgements This work has been performed within the European COST action MP 1205 Advances in optofluidics: integration of optical control and photonics with microfluidics.
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